Fiber-based lasers are well known for their usefulness in generating ultra-short optical pulses. In particular, the use of a non-linear loop mirror (i.e., figure-8) design as a fast saturable absorber in a fiber-based laser cavity is a well-established technique for ultra-short pulse generation.
However, the figure-8 design typically requires the use of some type of external perturbation to kick-start the modelocked operation of the laser cavity as required for generating a continuous train of optical pulses. “Self-starting” operation (that is, avoiding the need for manual manipulation of the laser to initiate modelocking) usually requires additional functionality of some type, such as some sort of modulation at the cavity repetition rate, intentional design of the cavity architecture to include more rigorous phase control, or an additional pump diode to balance the gain between the two loops. To date, achieving self-starting modelocking without adding extra (typically expensive) components to the basic figure-8 design has been difficult to achieve.
Moreover, not all possible modes generated by modelocking within the figure-8 structure are equally desirable, and there have been instances where a figure-8 laser will mode-lock into an undesirable mode. For example, attempts at dispersion management within the fiber cavity (a desirable goal) may bring many different regimes and physical effects into play, producing pulses that behave as solitons, dissipative solitons, stretched pulses, similaritons, and the like. Some of these modes might be desirable in certain cases, while others will be unsuited, depending on the application. Moreover, there exist a large variety of modes that will lock, but display large variations in phase and amplitude from one roundtrip through the figure-8 laser cavity to the next (referred to as “noisy” modes). For most applications, pulses that are too variable in amplitude or repetition rate are not stable enough to be a pulse train source.